The present application is based on Japanese priority applications No. 2000-146849 and 2000-336066 respectively filed on May 18, 2000 and Nov. 2, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to a quantum semiconductor device having quantum dots. Further, the present invention relates to optical detectors and optical memory devices that use such quantum dots.
In the field of optical telecommunication, various optical detectors are used for detecting optical signals transmitted through an optical fiber. Such optical detectors include high-speed PIN photodiodes. With recent increase of signal traffic in such optical telecommunication systems, on the other hand, there is a demand for faster optical detectors that are capable of operating with low power consumption and high photosensitivity.
In order to deal with sharply increasing optical traffic in such optical telecommunication systems, the use of so-called wavelength-multiplexing technology is spreading in the art of optical telecommunication. In such a wavelength-multiplexing technology, there is a demand for optical detectors that are capable of tuning to various optical wavelengths.
Conventionally, photodiodes having a p-n junction or a pin junction have been used extensively in optical telecommunication systems for high-speed detection of optical signals.
FIG. 1 shows the construction of a typical conventional p-n junction photodiode 10.
Referring to FIG. 1, the photodiode 10 is constructed on a substrate 11 of n-type InP, and includes a buffer layer 12 of n-type InP formed on the substrate 11, an optical absorption layer 13 of nxe2x88x92-type InGaAs formed on the buffer layer 12, and a p+-type region 13A of InGaAs formed inside the optical absorption layer 13, wherein the optical absorption layer 13 carries thereon an electrode 14 in correspondence to the p+-type region 13A. Further, a ring-shaped electrode 16 having an opening acting as an optical window is provided on a bottom principal surface of the InP substrate 11. Further, the exposed top surface of the optical absorption layer 13 is protected by an SiN passivation film 15. A photodiode having a pin junction also has a similar structure.
In view of the fact that the photodiode 10 of FIG. 1 has a planar construction, it is necessary in such a photodiode 10 to introduce incoming optical radiation over a substantial area in order to detect the optical current with sufficient S/N ratio. In other words, the photodiode 10 of FIG. 1 has a drawback of low sensitivity.
In the photodiode 10 of FIG. 1, it is possible to reduce the optical power needed for the photodiode to carry out the photodetection, by decreasing the optical area, in other words the area of the p+-type region 13A. However, such a decrease of the optical area is limited by the photolithographic process, and there arises a problem in that the photodiode 10 having the planar structure suffers from the problem of low sensitivity of photodetection.
In the field of information technology, on the other hand, there is occurring a sharp increase of data to be processed, and there is growing a need for a high-speed, large-capacity memory device for dealing with such large amount of data. When such a high-speed, large-capacity memory device is to be realized by a conventional semiconductor memory device, there arises a need of providing a very complex wiring pattern, and the complexity of the wiring pattern increases with increasing integration density of the semiconductor memory device. Associated therewith, it is expected that various problems, such as signal delay, decrease of yield and increase of cost, would be caused when such a memory device is constructed by conventional semiconductor memory devices.
In this regard, optical semiconductor memory devices that are written with information by a feeble optical signal are expected as being a device capable of overcoming the foregoing problems of semiconductor memory devices. By using optical semiconductor memory devices, it is expected to carry out writing and reading of information directly by an optical beam that can carry a large amount of information.
Conventionally, there is proposed a photo-electron integrated device that uses discrete energy levels characteristic to quantum dots for detection of optical signals as described in the Japanese Laid-Open Patent Publication 8-32046. According to this prior art, a number of quantum dots are formed in a planar structure and the quantum dots are used for optical detection, optical modulation, or optical output of wavelength-multiplex optical signals.
In such optical signal-processing device that uses quantum dots, it is necessary to form each of the quantum dots such that the quantum dot is tuned to the wavelength of the optical signal component to be detected. However, it is extremely difficult to form the desired quantum dots with necessary precision, quality and yield as long as conventional patterning process is used. Further, it is difficult to form the quantum dots with necessary size.
Accordingly, it is a general object of the present invention to provide a novel and useful quantum semiconductor device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a high-sensitivity optical detector.
Another object of the present invention is to provide an optical semiconductor memory device that is capable of being written with information by a feeble optical beam.
Another object of the present invention is to provide a quantum semiconductor device having quantum dots wherein the energy level of the quantum dots can be controlled as desired.
Another object of the present invention is to provide a spectrum analyzer that uses a quantum semiconductor device including therein quantum dots.
Another object of the present invention is to provide an optical receiver of wavelength-multiplex optical signals for selectively detecting an optical signal component in an incoming wavelength-multiplex optical signal.
Another object of the present invention is to provide a semiconductor optical detector, comprising:
a first semiconductor layer having a first conductivity type;
a second semiconductor layer formed on said first semiconductor layer;
a third semiconductor layer formed on said second semiconductor layer, said third semiconductor layer having said first conductivity type;
a pyramidal pit formed in said third semiconductor layer so as to invade into said second semiconductor layer, said pyramidal pit being defined by a plurality of facets merging together at an apex located in said second semiconductor layer in the vicinity of an interface to said first semiconductor layer, said apex being located with an offset from said interface;
a channel layer covering said plurality of facets continuously in said pyramidal pit, said channel layer having said first conductivity type;
a barrier layer formed in said pyramidal pit so as to cover said channel layer;
a cap layer formed in said pyramidal pit so as to cover said barrier layer;
an optical window formed on said cap layer;
a first electrode formed on said third semiconductor layer; and
a second electrode formed on said first semiconductor layer.
Another object of the present invention is to provide a semiconductor optical detector, comprising:
a substrate having a first conductivity type;
a first semiconductor layer formed on said substrate, said first semiconductor layer having said first conductivity type;
a second semiconductor layer formed on said first semiconductor layer;
a third semiconductor layer formed on said second semiconductor layer, said third semiconductor layer having said first conductivity type;
a pyramidal pit formed in said third semiconductor layer so as to invade into said second semiconductor layer, said pyramidal pit being defined by a plurality of facets merging together at an apex located in said second semiconductor layer in the vicinity of an interface to said first semiconductor layer, said apex being located with an offset from said interface;
a channel layer covering said plurality of facets continuously in said pyramidal pit, said channel layer having said first conductivity type;
a barrier layer formed in said pyramidal pit so as to cover said channel layer;
a cap layer formed in said pyramidal pit so as to cover said barrier layer;
a first electrode formed on said third semiconductor layer; and
a second electrode formed on a bottom principal surface of said substrate,
said second electrode having an optical window.
According to the present invention, an optical beam coming incident through the optical window induces an optical excitation of electron-hole pairs in the channel layer, and a photocurrent flows through the channel layer. As the channel layer covers the facets of the pyramidal pit, the photocurrent flows through the channel layer from the first electrode to the second electrode on each of the foregoing facets. Thus, the photocurrents flowing through the channel layer are collected at the apex of the pyramidal pit and are transferred therefrom to the second electrode. Thus, the optical detector of the present invention can detect the photocurrent even in such a case the optical power of the incoming optical beam is very weak.
Another object of the present invention is to provide an optical semiconductor memory device, comprising:
a first semiconductor layer having a first conductivity type;
a second semiconductor layer formed on said first semiconductor layer;
a third semiconductor layer formed on said second semiconductor layer, said third semiconductor layer having said first conductivity type;
a pyramidal pit formed in said third semiconductor layer so as to invade into said second semiconductor layer, said pyramidal pit being defined by a plurality of facets merging together at an apex located in said second semiconductor layer in the vicinity of an interface to said first semiconductor layer, said apex being located with an offset from said interface;
a channel layer covering said plurality of facets continuously in said pyramidal pit, said channel layer having said first conductivity type;
a first barrier layer formed in said pyramidal pit so as to cover said channel layer continuously, said first barrier layer having a second conductivity type;
a hole-accumulation layer formed on said first barrier layer in said pyramidal pit;
a second barrier layer formed in said pyramidal pit so as to cover said hole-accumulation layer, said second barrier layer having said second conductivity type;
a cap layer formed in said pyramidal pit so as to cover said second barrier layer;
an optical window formed on said cap layer;
a first electrode formed on said third semiconductor layer; and
a second electrode formed on said first semiconductor layer.
Another object of the present invention is to provide an optical semiconductor memory device, comprising:
a substrate having a first conductivity type;
a first semiconductor layer formed on said substrate, said first semiconductor layer having said first conductivity type;
a second semiconductor layer formed on said first semiconductor layer;
a third semiconductor layer formed on said second semiconductor layer, said third semiconductor layer having said first conductivity type;
a pyramidal pit formed in said third semiconductor layer so as to invade into said second semiconductor layer, said pyramidal pit being defined by a plurality of facets merging together at an apex located in said second semiconductor layer in the vicinity of an interface to said first semiconductor layer, said apex being located with an offset from said interface;
a channel layer covering said plurality of facets continuously in said pyramidal pit, said channel layer having said first conductivity type;
a first barrier layer formed in said pyramidal pit so as to cover said channel layer continuously, said first barrier layer having a second conductivity type;
a hole-accumulation layer formed on said first barrier layer in said pyramidal pit;
a second barrier layer formed in said pyramidal pit so as to cover said hole-accumulation layer, said second barrier layer having said second conductivity type;
a cap layer formed in said pyramidal pit so as to cover said second barrier layer;
a first electrode formed on said third semiconductor layer; and
a second electrode formed on a bottom principal surface of said substrate,
said second electrode having an optical window.
According to the present invention, an optical beam incident through the optical window induces an optical excitation of electron-hole pairs. Thereby, the holes thus excited are retained in a potential well formed in the hole-accumulation layer, while the electrons reach the channel layer by overriding the first barrier layer. The electrons thus reached the channel layer are absorbed by the second electrode. As a result, there occurs an accumulation of holes in the hole-accumulation layer and the conduction band and the valence band of the hole-accumulation layers are shifted in the lower energy side with respect to the channel layer. In response to such a change of the band energy, the carrier density in the channel layer is increased and the electric current flowing through the channel layer is changed in response to the number of the holes accumulated in the hole-accumulation layer. Thus, the optical semiconductor memory device of the present invention can read out the information written optically into the hole-accumulation layer in the form of holes by detecting the electric current flowing between the first and second electrodes. In order to achieve a reliable retention of holes in the hole-accumulation layer, it is preferable to form a quantum dot in the hole-accumulation layer in the vicinity of the apex of the pyramidal pit. By providing a transparent electrode in correspondence to the optical window, it is possible to erase the written information by injecting electrons from the transparent electrode such that the accumulated holes are annihilated by the injected electrons.
Another object of the present invention is to provide a quantum semiconductor device, comprising:
a first semiconductor layer having a first conductivity type;
a second semiconductor layer formed on said first semiconductor layer;
a third semiconductor layer formed on said second semiconductor layer, said third semiconductor layer having said first conductivity type;
a pyramidal pit formed in said third semiconductor layer so as to invade into said second semiconductor layer, said pyramidal pit being defined by a plurality of facets merging together at an apex located in said second semiconductor layer in the vicinity of an interface to said first semiconductor layer, said apex being located with an offset from said interface;
a channel layer covering said plurality of facets continuously in said pyramidal pit, said channel layer having said first conductivity type;
a first barrier layer formed in said pyramidal pit so as to cover said channel layer continuously, said first barrier layer having a second conductivity type;
a hole-accumulation layer formed on said first barrier layer in said pyramidal pit;
a second barrier layer formed in said pyramidal pit so as to cover said hole-accumulation layer, said second barrier layer having said second conductivity type;
a cap layer formed in said pyramidal pit so as to cover said second barrier layer;
an optical window formed on said cap layer;
a first electrode formed on said first semiconductor layer; and
a second electrode formed on said second semiconductor layer in Schottky contact therewith.
According to the present invention, the size of the quantum dot formed in the hole-accumulation layer in correspondence to the apex of the pyramidal pit is changed in response to a control voltage applied to the second electrode. It should be noted that the control voltage applied to the second electrode changes the extension of the depletion layer associated with the Schottky contact and hence the size of the quantum dot. With the change of the quantum dot, the quantum levels associated with the quantum dot are also changed.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.